It's probably dry. The skin on the face is thinner than the rest of the body, so it's more susceptible to being dry. Try a light lotion, right after he washes it or showers - it works better if the skin is still a little damp. It might sting the first time or two - because if it is dry enough to itch, it might be dry enough to be damaged, too. But it will feel much better, if he does it consistently.
Also, if it just itches around his mouth - how messy is he, and does he eat spicy food? My daughter had this - pepperoni didn't give her a reaction when she ingested it, but it did give her a contact allergy, if she was a messy eater, and got it on her face.